Effect of M1 Matrix Protein on Hemagglutinin Expression and Influenza Virus-Like Particle Production in Trichoplusia NI

Virus-like particles (VLPs) are a new and effective flu vaccine approach. VLPs are non-infectious and can produce robust immune responses. Flu VLPs are typically created by co-expressing the flu hemagglutinin (HA) surface antigen and the matrix protein M1-which assists in viral and VLP budding-in cultured cells; however, there may be theoretical advantages to reducing or eliminating the M1 component of flu VLP vaccines. Reducing or eliminating M1 may result in higher HA expression, and therefore more efficient vaccine production. This project examined the effect of M1 matrix protein co-expression on HA expression and VLP production in a Trichoplusia ni living bioreactor expression system. T. ni pupae were injected with varying doses and ratios of influenza HA and M1 baculoviruses. After several days of incubation, a crude extract was obtained from the pupae and a VLP-enriched particulate fraction prepared by two-step centrifugation. We found that exclusion of M1 does not preclude formation of VLP-like particles, and that HA expression increased as the dose of M1 decreased. We also found that lower doses of infecting baculoviruses correlated with higher HA expression. Transmission electron microscopy studies were used to investigate the effects of matrix M1 content on VLP morphology. VLP morphology was independent of M1 dose. The optimization of HA expression through HA:M1 ratio and total dose will enable production of VLPs for pre-clinical evaluation of broadly-protective flu vaccine candidates.